A spark plug is installed to an internal combustion engine (engine) and the like and used for igniting the air-fuel mixture and the like inside a combustion chamber. In general, the spark plug includes an insulator having an axial hole extending along the axial direction, a center electrode inserted in the tip side of the axial hole, a metal shell provided to the outer circumference of the insulator, and a ground electrode fixed to the tip portion of the metal shell. Further, a gap is formed between the tip portion of the ground electrode and the tip portion of the center electrode, and the ignition to the air-fuel mixture and the like is made by applying a high voltage to the center electrode (gap) to generate a spark discharge.
Further, in recent years, high compression and high supercharging engines have been proposed for the improved fuel economy and the like. In such engine, the in-cylinder pressure is relatively high, which requires a higher voltage (discharge voltage) for generating the spark discharge (for example, 37 kV or more). Therefore, when the voltage for generating the spark discharge is applied to the center electrode, a discharge penetrating the insulator between the center electrode and the metal shell (a penetration discharge) is likely to occur and thus there is a likelihood that the spark discharge cannot be properly generated.
Therefore, in order to improve the dielectric strength of the insulator, an approach has been proposed wherein the thickness of the tip portion of the insulator is increased, i.e., is formed relatively thick and where the spark penetration is particularly likely to occur (see JP-A-2000-243535, for example).
When the tip of the insulator is thickened, however, a large thermal shock occurs at the tip portion of the insulator at a heating and cooling, which is likely to cause a breakage of the insulator. In particular, in a direct injection engine where the fuel is directly injected to the tip portion of the insulator, breakage of the insulator due to the thermal shock is of greater concerned, because the insulator is rapidly cooled by the fuel.
The present invention is made taking the above situation into consideration and its purpose is to provide a spark plug that is able to effectively suppress the breakage of the insulator due to the thermal shock while further ensuring the prevention of the spark penetration in the insulator.
Below, each configuration suitable to solve the above-described problem will be described by listing items. It is noted that the effect and advantage specific to the corresponding configuration will be additionally described, if necessary.